1. Field of the Invention
This invention relates to the field of digital-to-analog converters (DACs), and particularly to circuits and methods for automatically calibrating the current sources making up a switched current source DAC.
2. Description of the Related Art
Demand for high speed/high resolution DACs continues to grow, driven primarily by strong growth in the markets for wired and wireless communications. One architecture which has been used to construct high speed/high resolution DACs employs an array of current sources: the DAC receives a digital input word which represents a desired output current, and the current sources are selectively switched to an output to provide the desired output current. Such "switched current source" DACs have been favored for high speed and high resolution applications due to their ability to drive a resistive load directly, without the need for a voltage buffer.
One problem which afflicts switched current source DACs is current source mismatch. A typical switched current source DAC employs a segmented current source design, with the DAC's most significant bits (MSBs), upper least significant bits (ULSBs), and lower least significant bits (LLSBs) implemented with respective current source subarrays, with the current sources in a given subarray ideally producing identical output currents. Mismatch between the current sources in a given subarray, particularly if within the MSB subarray, degrades the DAC's static linearity, which in turn degrades its dynamic linearity.
Some method of static calibration is typically employed to reduce mismatch between current sources. Traditionally, as discussed, for example, in D. Groeneveld et al., "A Self-Calibration Technique for Monolithic High-Resolution D/A Converters,", IEEE Journal of Solid-State Circuits, vol. 24, pp. 1517-1522, December 1989, this has been accomplished by using an additional current source within a subarray, which allows one current source to be taken out of the circuit at any one time for calibration measurement and/or correction purposes, leaving a full bank of current sources available for normal DAC operation.
The requirement for an additional current source, however, can impact the DAC's dynamic performance, since the switching in and out of current sources at the calibration rate introduces spurs at the calibration frequencies in the DAC output spectrum. In practice, these spurs include additional dynamic components from dynamic mismatches between the current source arrays being used at any one time. These dynamic mismatches are not attenuated by the calibration mechanism, and can, in fact, be increased by mismatches in the switches needed to facilitate the calibration.